The hydroformylation reaction is well known in the art as a catalytic method for the conversion of an olefin into an aldehyde product having one carbon more than the starting olefin by the addition of one molecule each of hydrogen and carbon monoxide to the carbon-carbon double bond. If the organic substrate contains more than one carbon-carbon double bond, more than one formyl group can be added to the substrate, thereby increasing the number of carbon atoms contained in the product molecule by more than one.
Industrial processes for the catalytic hydroformylation of higher olefins (i.e., those olefins having more than five carbons) face several challenges, including efficient catalyst recovery/recycle and the limited solubilities of the gaseous reactants (H2 and CO) in the liquid reaction phase. See Frohling et al., Applied homogeneous catalysis with organometallic compounds, VCH, Weinheim, Germany, 27-104 (1996). The commercial catalysts used in the lower olefin processes, mostly rhodium-based, are not applied in higher olefin hydroformylation because of their instability at the temperatures required for product separation/distillation. Hence, while the less expensive cobalt-based catalysts are used, harsher conditions (140-200° C., 5-30 MPa) are often employed to activate and stabilize the catalysts. In addition, the catalyst recovery typically involves significant quantities of solvents, acids, and bases in a series of many operating units. See Garton et al., PCT International Application, WO 2003/082789. Thus, an engineered system is desired to realize process intensification at milder conditions with a highly active catalyst that requires a relatively simpler and environmentally friendlier catalyst recovery method.
Several approaches for catalyst recovery have been reported in literature. The first approach involves immobilizing homogeneous rhodium (“Rh”) catalysts on various supports, i.e., the silicate MCM-41 (see Marteel et al., Supported platinum/tin complexes as catalysts for hydroformylation of 1-hexene in supercritical carbon dioxide, Catalysis Communications, 4 309-314 (2003)), zeolites (see Mukhopadhyay et al., Encapsulated HRh(CO)—(PPh3)3 in microporous and mesoporous supports: novel heterogeneous catalysts for hydroformylation, Chemical Materials, 15 1766-1777 (2003)), nanotubes (see Yoon et al., Rh-based olefin hydroformylation catalysts and the change of their catalytic activity depending on the size of immobilizing supporters, Inorganica Chimica Acta., 345 228-234 (2003)), supported aqueous phase catalysis (“SAPC”) (see Dessoudeix et al., Apatitic tricalcium phosphate as novel smart solids for supported aqueous phase catalysis (SAPC), Advanced Synthetic Catalysis, 344 406-412 (2002)), and polymers (see Lu et al., Hydroformylation reactions with recyclable rhodiumcomplexed dendrimers on a resin, Journal of American Chemical Society, 125 13126-13131 (2003) and Lopez et al., Evaluation of polymer-supported rhodium catalysts in 1-octene hydroformylation in supercritical carbon dioxide, Industrial & Engineering Chemistry Research, 42 3893-3899 (2003)).
The second approach involves biphasic media, such as water/organic (see Peng et al., Aqueous biphasic hydroformylation of higher olefins catalyzed by rhodium complexes with amphiphilic ligands of sulfonated triphenylphosphine analog, Catalysis Letters, 88 219-225 (2003)), water/CO2 (see Haumann et al., Hydroformylation in microemulsions: conversion of an internal long chain alkene into a linear aldehyde using a water soluble cobalt catalyst, Catalysis Today, 79-80 43-49 (2003); McCarthy et al., Catalysis in inverted supercritical CO2/aqueous biphasic media, Green Chemistry, 4(5) 501-504 (2002)), and room temperature ionic liquid/CO2 (see Webb, Continuous flow hydroformylation of alkenes in supercritical fluid-ionic liquid biphasic systems, Journal of American Chemical Society, 125 15577-15588 (2003)), wherein the catalyst is sequestered in either the water or the ionic liquid phases whereas the product preferentially separates into the organic phase or the CO2 phase.
The third approach involves employing a “phase transition switch” whereby reactions are performed homogeneously, following which the catalysts are recovered from the product stream via phase transition triggered by a change in either the system temperature (see Horváth et al., Facile catalyst separation without water: fluorous biphasic hydroformylation of olefins, Science, 266 (5182) 72-75 (1994); Zheng et al., Thermoregulated phase transfer ligands and catalysis. III. Aqueous/organic two-phase hydroformylation of higher olefins by thermoregulated phase-transfer catalysis, Catalysis Today, 44 175-182 (1998)) or pressure (see Koch et al., Rhodium-catalyzed hydroformylation in supercritical carbon dioxide, Journal of American Chemical Society, 120 13398-13404 (1998); Palo et al., Effect of ligand modification on rhodium-catalyzed homogeneous hydroformylation in supercritical carbon dioxide, Organometallics, 19 81-86 (2000)).
The use of CO2-expanded liquids (“CXLs”) as reaction media has received increased attention by the present inventors. CXLs are a continuum of compressible media generated when various amounts of dense phase carbon dioxide are added to an organic solvent. CXLs offer both reaction and environmental benefits. Near-critical carbon dioxide possesses highly tunable transport properties ranging from gas-like diffusivities to liquid-like viscosities. See Subramaniam et al., Reaction in supercritical fluids—a review, Industrial & Engineering Chemistry Process Design and Development, 25 1-12 (1986). The presence of dense CO2 imparts similar tunability to CXLs as well. The solubilities of many gaseous reagents (i.e., O2, H2) in CXLs are enhanced several-fold relative to the neat liquid phase (i.e., those without any CXLs). See Hert et al., Enhancement of oxygen and methane solubility in 1-hexyl-3-methylimidazolium bis(trifluoromethylsul-fonyl)imide using carbon dioxide, Chemical Communications, 2603-2605 (2005); Wei et al., Autoxidation of 2,6-di-tertbutyl-phenol with cobalt Schiff base catalysts by oxygen in CO2-expanded liquids, Green Chemistry, 6 387-393 (2004); Solinas et al., Enantioselective hydrogenation of imines in ionic liquid/carbon dioxide media, Journal of American Chemical Society, 126 16142-16147 (2004); Bezanehtak et al., Vapor-liquid equilibrium for the carbon dioxide+hydrogen+methanol ternary system, Journal of Chemical Engineering Data, 49 430-434 (2004); Xie et al., Bubble and dew point measurements of the ternary system carbon dioxide+methanol+hydrogen at 313.2 K, Journal of Chemical Engineering Data, 50 780-783 (2005). Although most transition metal complexes are only sparingly soluble in supercritical CO2 (scCO2), the presence of an appropriate amount of the organic liquid in CXLs ensures adequate solubilities of transition metal complexes in a CXL phase for performing homogeneous catalysis. Further, such solubilities are realized at pressures an order of magnitude lower than those required in scCO2 medium for solubilizing Rh catalyst complexes with fluorinated ligands. See Palo et al., Effect of ligand modification on rhodium-catalyzed homogeneous hydroformylation in supercritical carbon dioxide, Organometallics, 19 81-86 (2000).
Recently, the present inventors reported the homogeneous catalytic hydroformylation of 1-octene in CO2-expanded acetone with an unmodified rhodium catalyst. See Jin et al. Homogeneous catalytic hydroformylation of 1-octene in CO2-expanded solvent media, Chemical Engineering Science, 59 4887-4893 (2004). At 30 and 60° C., the turnover frequencies (“TOFs”) in CO2-expanded acetone were up to four-fold greater than those obtained in either neat acetone (a polar solvent) or compressed CO2. The enhanced rates in CXLs were realized at significant solvent replacement (up to 80% by volume) and at mild operating pressures (less than 12 MPa). Although the hydroformylation rates were enhanced, the regioselectivity towards linear and branched aldehydes (n/i ratio) remained unaffected by the change in either the acetone/CO2 ratio or the temperature.
In industrial practice, the ability to tune the product regioselectivity is of interest. In the case of industrial linear olefin hydroformylation, the linear aldehyde is the desired product. It is shown in the present invention, that when the properties of the reaction mixture (containing an excess of non-polar 1-octene as the solvent) are tuned by CO2 addition, the regioselectivity may be tuned. At increased CO2 content in the liquid phase reaction mixture, the regioselectivity is favored toward the linear product and vice-versa. With change in CO2 content, the syngas solubilities in and the dielectric constant of the resulting reaction mixture may be continuously varied, influencing the product selectivity. This aspect was not observed in the previous published literature which described experiments performed with a polar solvent such as acetone mixed with CO2.